perf: complete storage-layer UUID migration — eliminate UUID↔String round-trips

- Domain: Folder.owner_id + File.owner_id → Option<Uuid> (was Option<String>)
- Traits: folder_repository signatures accept Uuid directly (was &str)
- Infra: remove all fi.user_id::text / fo.user_id::text SQL casts in repos
- Infra: remove $N::uuid bind casts (columns are native UUID now)
- Services: eliminate to_string() bridge variables in folder_service,
  search_service, trash_service
- DTOs: boundary conversion to String only at API edge (file_dto, folder_dto)
- Ports: default impls compare Option<Uuid> directly
- Stubs: updated signatures to match trait

13 files changed across domain/application/infrastructure layers.
Compiles clean (cargo check passes).

src/application/services/trash_service.rs

@@ -260,7 +260,7 @@ impl TrashUseCase for TrashService {
 
                 // Ownership check — return NotFound (not Forbidden) to
                 // prevent leaking whether the folder exists.
-                if folder.owner_id().is_none_or(|o| o != user_id.to_string()) {
+                if folder.owner_id() != Some(user_id) {
                     return Err(DomainError::not_found(
                         "Folder",
                         format!("Folder not found: {}", item_id),